1. Field of the Invention
The invention relates generally to techniques and compositions which provide improved hardfacing materials. In particular, the present invention relates to compositions for use in hardfacing milled-teeth bits.
2. Background Art
Bits for drilling oil wells (e.g., “rock bits”) typically have steel bodies (“bit bodies”) that are connected at the bottom of drill strings. One or more roller cones are rotatably mounted to the bit body. These roller cones have a plurality of teeth attached thereto. These teeth crush, gouge, and scrape rock at the bottom of a hole being drilled. Several types of roller cone drill bits are available for drilling wellbores through earth formations, including insert bits (e.g., tungsten carbide insert bit—“TCI”) and milled tooth bits. The following illustration and discussion will use the milled tooth bits as examples. It should be noted, however, that the invention is not limited to this type of bits. Instead, the invention is applicable to any rock bit or drag bit.
Milled tooth bits include one or more roller cones rotatably mounted to a bit body. The one or more roller cones are typically made from steel and include a plurality of teeth formed integrally with the material from which the roller cones are made. Typically, a hardfacing material is applied, such as by arc or gas welding, to the exterior surface of the teeth to improve the wear resistance of the teeth. The hardfacing material typically includes one or more metal carbides, which are bonded to the steel teeth by a metal alloy (“binder alloy”). In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface of the teeth. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal provides fracture toughness to the hardfacing.
Many factors affect the durability of a hardfacing composition in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
The metal carbide most commonly used in hardfacing is tungsten carbide. Small amounts of tantalum carbide and titanium carbide may also be present in such material, although these other carbides may be considered to be deleterious.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Three types of tungsten carbide commonly used in hardfacing drill bits are cast tungsten carbide, macro-crystalline tungsten carbide, and cemented tungsten carbide (also known as sintered tungsten carbide).
Cemented tungsten carbide refers to a material formed by mixing particles of tungsten carbide, typically monotungsten carbide, and particles of cobalt or other iron group metal, and sintering the mixture. In a typical process for making cemented tungsten carbide, small tungsten carbide particles, e.g., 1-15 microns, and cobalt particles are vigorously mixed with a small amount of organic wax which serves as a temporary binder. An organic solvent may be used to promote uniform mixing. The mixture may be prepared for sintering by either of two techniques: it may be pressed into solid bodies often referred to as green compacts; alternatively, it may be formed into granules or particles such as by pressing through a screen, or tumbling and then screened to obtain more or less uniform particle size.
Such green compacts or particles are then heated in a vacuum furnace to first evaporate the wax and then to a temperature near the melting point of cobalt (or the like) to cause the tungsten carbide particles to be bonded together by the metallic phase. After sintering, the compacts are crushed and screened for the desired particle size. Similarly, the sintered particles, which tend to bond together during sintering, are gently churned in a ball mill with media to separate them without damaging the particles. Some particles may be crushed to break them apart. These are also screened to obtain a desired particle size. The crushed cemented carbide is generally more angular than the particles which tend to be rounded.
Another type of tungsten carbide is macro-crystalline carbide. This material is essentially stoichiometric tungsten carbide created by a thermite process. Most of the macro-crystalline tungsten carbide is in the form of single crystals, but some bicrystals of tungsten carbide may also form in larger particles. Single crystal stoichiometric tungsten carbide is commercially available from Kennametal, Inc., Fallon, Nev.
Carburized carbide is yet another type of tungsten carbide. Carburized tungsten carbide is a product of the solid-state diffusion of carbon into tungsten metal at high temperatures in a protective atmosphere. Sometimes, it is referred to as fully carburized tungsten carbide. Such carburized tungsten carbide grains usually are multi-crystalline, i.e., they are composed of tungsten carbide agglomerates. The agglomerates form grains that are larger than the individual tungsten carbide crystals. These large grains make it possible for a metal infiltrant or an infiltration binder to infiltrate a powder of such large grains. On the other hand, fine grain powders, e.g., grains less than 5 μm, do not infiltrate satisfactorily. Typical carburized tungsten carbide contains a minimum of 99.8% by weight of tungsten carbide, with a total carbon content in the range of about 6.08% to about 6.18% by weight.
Regardless of the type of hardfacing material used, designers continue to seek improved properties (such as improved wear resistance, thermal resistance, etc.) in the hardfacing materials. Unfortunately, increasing wear resistance usually results in a loss in toughness, or vice-versa. One suggested technique has been to apply a “coating” layer around a hardfacing granule.
U.S. Pat. No. 6,138,779 (the '779 patent) discloses a technique of coating cubic boron nitride particles with tungsten carbide particles or a ceramic to provide a thermal barrier to prevent thermal degradation of the particle into a reduced wear resistant phase.
In particular, the '779 patent discloses a hardfacing composition to protect wear surfaces of drill bits and other downhole tools that consists of coated cubic boron nitride particles or coated particles of other ceramic, superabrasive or superhard materials dispersed within and bonded to a matrix deposit. The coating on the ceramic particles or particles of other hard materials may be formed from materials and alloys such as tungsten carbide, and tungsten carbide/cobalt and cermets such as metal carbides and metal nitrides. The coated particles are preferably sintered and have a generally spherical shape. The coated particles are pre-mixed with selected materials such that welding and cooling will form both metallurgical bonds and mechanical bonds within the solidified matrix deposit.
A welding rod may be prepared by placing a mixture of selected hard particles such as coated cubic boron nitride particles, hard particles such as tungsten carbide/cobalt, and loose filler material into a steel tube. A substrate may be hardfaced by progressively melting the welding rod onto a selected surface of the substrate and allowing the melted material to solidify and form the desired hardfacing with coated cubic boron nitride particles dispersed within the matrix deposit on the substrate surface.
However, while this coating may improve the thermal resistance, the hardfacing composition has in service limitations due to the brittle nature of the superhard family of materials, including boron nitride and silicon nitrite.